This document provides an overview on how and why EGR coolers are utilized, defines commonly used nomenclature, discusses design issues and trade-offs, and identifies common failure modes. The reintroduction of exhaust gas into the combustion chamber is just one component of the emission control strategy for internal combustion (IC) engines, both diesel and gasoline, and is useful in reducing exhaust port emission of Nitrogen Oxides (NOx). Other means of reducing NOx exhaust port emissions are briefly mentioned, but beyond the scope of this document.